ASTM standard C167-98 specifies standard methods for measuring the thickness and density of blanket or batt thermal insulation. The standard specifies use of a depth gage such as gage 600 shown in FIG. 6. The depth gage 600 has a clear plastic disk 610 of about 76 mm (3 inches) attached near or at a top end of a pin 620 having sufficient length for the material to be measured. The disk 610 is aligned perpendicular to the pin 620. A friction device is used to fix the disk to the pin. For example, the disk 610 may have a hub 612 with a female threaded sleeve 614 to receive a set screw 616. The set screw 616 can be advanced to engage a side surface of the pin 620. The pin 620 has a sharpened point 620p. 
To use the depth gage 600, the disk is placed at the top end of the pin 620, as shown. The pin 620 is pushed into and through the insulation (not shown in FIG. 6), until the pin contacts a hard surface below the insulation. The set screw 616 is then loosened, so that the disk 610 can be freely lowered relative to the pin 610, until the disk contacts the top surface of the insulation. The contact is detected visually, by looking through the clear plastic disk. The set screw 616 is then tightened, and the gage 600 is removed. The distance between the bottom surface of disk 610 and the point 620p of pin 620 is the thickness of the insulation. A graduated rule (not shown) is placed against the disk 610, parallel to the pin. The distance is measured visually, by looking at where the point 620p lies along the rule.
The precision of the measurement method described above is limited by the precision of the rule, and the accuracy of the measurement is limited by any variation in the angle from which the test person views the pin 620 and rule while taking a reading. Also, during the initial step of contacting the insulation with the disk, the weight of the disk and any force inadvertently exerted by the test person may result in compression of the insulation, further reducing the accuracy of the measurement.
An improved method and apparatus for measuring insulation depth is desired.